The invention relates to a process for the mass-spectrometric investigation of isotopes, as well as an isotope mass spectrometer.
In a whole series of various sciences, the relative isotope frequencies of individual elements are used in the investigation of substances as indicators for specified set questions. Merely by way of examples, reference is made here to medicine, in which, for example, deuterium is used as a labelling substance and its relative frequency in relation to hydrogen is investigated, or geology, in which the determination of the age of rocks is carried out, for example, by means of a determination of the frequency of specified rubidium and strontium isotopes in the rock. In all cases, the matter of importance is to obtain exact information concerning the frequency of isotopes which differ with respect to their masses only by one single mass unit.
There are now a plurality of systematic error sources, which lead to a situation in which isotopes of a specified mass are attributed, in the course of the mass-spectroscopic investigation, to those which exhibit a different, neighbouring mass (i.e. are falsely recognised). By way of examples of such error sources, reference is made here, only by way of key words, to errors in the inlet system (separation of the masses by their flow velocity in the inlet system, errors in the ion source, separation of the masses by magnetic fields, probability of ionization, vaporization processes etc), errors in the separation system (e.g. homogeneity errors) or also error sources in the ion detection system. It is also possible for measurement errors to occur as a result of interactions between the ions (in the ion source or in the separator) or as a result of collision processes. These errors always act in the form of an incursion onto the neighbour mass; this is designated as abundance sensitivity.
In order to reduce the measurement error, arrangements are now known which exhibit, on the one hand, a sector magnet and, on the other hand, a downstream-situated electrostatic sector field, which serves to mask out ions which have lost energy on account of a scattering process. The incursions onto neighbour masses or abundance sensitivity can be improved, by this, to values of approximately 10.sup.-7.
A further improvement can be achieved by tandem mass spectrometers, in which two sector magnets are disposed one behind the other with oppositely directed deflection. In these arrangements, a pulse-controlled two-fold deflection of the ion curren takes place. The abundance sensitivity which can be achieved amounts to approximately 10.sup.-12.
In both cases, it is therefore necessary to effect a considerable expenditure on apparatus, in order to achieve an improvement of the accuracy of measurement.